The field of the invention is tubular connections, which are expanded downhole, and more particularly threads that retain makeup torque despite longitudinal shrinkage resulting from expansion downhole.
Oilfield tubular joints comprise a threaded pin and box which are made up against one or more torque shoulders with a force than can exceed 3000 foot pounds. Some designs further incorporate a resilient seal. One of the problems with such seals is that they fail because of a residual clearance in the seal area between the pin and the box, after makeup. When such joints are used downhole and expanded, longitudinal shrinkage at the pin end results in a pulling away of the pin end from the torque shoulder on the box. This allows very low torque levels to undo the connection with values as low as 200 foot-pounds or less. This longitudinal shrinkage can also eliminate the metal-to-metal seal, which existed after makeup and before expansion.
The present invention seeks to overcome, or at least alleviate, these shortcomings of the prior designs. It provides for a locking feature that retains the makeup torque and that is energized due to the expansion. It reduces or eliminates clearance around resilient seals due to the expansion. It also seeks to enhance the locking feature using spaced seals, which can trap atmospheric pressure between them, such that upon insertion downhole, hydrostatic pressure can push the box into the pin to better engage a locking feature. These and other advantages of the present invention can be more readily appreciated by a review of the detailed description of the preferred embodiment, which appears below.
A thread for expandable downhole tubular connections is disclosed. It features a locking pattern on the pin and box, which engages upon expansion of the connection. The locking pattern compensates for longitudinal shrinkage away from a torque shoulder, as a result of expansion. A resilient seal can be incorporated into the design with pin to box clearance disappearing around the seal as a result of expansion. Resilient seals can be used in tandem to trap atmospheric pressure between them. The wellbore hydrostatic pressure helps engage the locking feature by pushing the box into the pin, helped by the trapped low pressure between the seals.